The present invention relates to a calibration circuit for a band-gap reference voltage, particularly in integrated circuits implemented by BiCMOS technology.
Particular band-gap reference voltage circuits, adapted to provide a voltage, called band-gap reference voltage, for a good way of working of analog and integrated circuits are known to provide a reference as constant as possible with the change of the working temperature and of the supply voltage.
It is known from the physics of the semiconductor devices that the base-emitter voltage Vbe of a transistor decreases of about 2 millivolt each centigrade degree and by extrapolating the function until the zero Kelvin (0 K=xe2x88x92273.15xc2x0 C.) it gets that Vbe of silicon transistors is 1.205 V, that is exactly the band-gap reference. Therefore by summing to said band-gap voltage a voltage which grows with the same law with the increase of the temperature, the resulting voltage remains constant at the value of the band-gap voltage.
Band-gap reference circuits generally use two transistors implemented by bipolar transistors, working at different current values, and said circuits use further devices adapted to develop a voltage proportional to the difference of the two base-emitter voltages, called xcex94Vbe, of said transistors.
In the prior art, in order to obtain this result, the bases of the two transistors are usually connected to each other and a resistance connects their emitter in such a way as to detect the differences between the Vbes, so that it is possible to obtain a band-gap reference voltage as a weighted sum of voltages having opposite temperature coefficients, that is:
Vbandgap=Vbe+G*xcex94Vbe
wherein xe2x80x9cVbexe2x80x9d is the negative coefficient of the voltage of the base-emitter junction of a bipolar transistor and xe2x80x9cG *xcex94Vbexe2x80x9d is a voltage having a positive temperature coefficient. In this way the resulting voltage, that is the reference or Vbandgap voltage, has a temperature coefficient equal to zero in the ideal case.
Conventional circuits of band-gap voltage reference are described in many technicalxe2x80x94scientific literature books, including for instance P. R. Gray and R. G. Meyer, xe2x80x9cAnalysis and Design of Analog Integrated Circuit, Third Editionxe2x80x9d, McGraw Hill, 1994.
However imperfections of process can compromise the right way of working of the reference circuit and in particularly the equality of the two resistances used as weight, as well as the realization of other circuit devices necessary for the working of the band-gap reference circuit, such as, for example, a current mirror, is extremely delicate
In the U.S. Pat. No. 4,808,908 in order to avoid the fluctuations of the band-gap voltage, as a consequence of process spreads, said voltage is regulated by means of resistances (resistances of devices such as, for example, Zenerxe2x80x94zap or poly fuse) adapted to stabilize the output reference voltage of the circuit, and calibrated by the use of a laser.
In the U.S. Pat. No. 5,325,045 the band-gap voltage is regulated by means of a modification of the ratio of the number of the emitter electrodes, using more bipolars in parallel, so as to generate axcex94Vbe.
In view of the state of the art described, it is an object of the present invention to make insensitive the band-gap reference circuit to the working temperature changes and to the supply voltage fluctuations so as to guarantee a better calibration of the reference voltage. According to the present invention, such object is achieved by a calibration circuit for a band-gap voltage comprising first and second transistors working at different current density, having the base electrodes connected to each other, a first resistance connecting the emitter electrodes of said first and second transistors, said first transistor having a second resistance in series to its emitter electrode, said first and second transistors being connected with a circuitry of transistors, configured as a mirror, characterized by comprising a current source, generating a current in function of the value present in a digital word, composed by xe2x80x9cixe2x80x9d bit, connected by means of first and second switches to respective first and second circuit nodes so as to select in which node to insert the current and so as to select the necessary quantity of current to make the calibration.
Thanks to the present invention it is possible to make a circuit able to minimize the area dissipation by reducing the number of used resistances.
Furthermore it is possible to make a circuit having at disposition a higher number of calibration levels with equal used bits.
It is also possible to make a circuit having a temperature behavior which is easily detectable and controllable, because any device is inserted in the electric path, adapted to generate the reference voltage.